In general everyday use, railcars collide together frequently. For example, the railcars of a train in motion generally bump into each other when the train slows or stops. Also, railcar collisions occur when assembling railcars into a train. The difference in velocity between the railcars in such collisions is typically low. However, due to the large mass of the railcars, the railcars collide with sufficient impact energy, unless otherwise absorbed, to cause damage to the railcars and any cargo carried by the railcars even in these collisions at low velocity differences. To absorb the impact of normal railcar collisions, a railcar generally includes cushion units mounted at each end of the railcar between the railcar and its couplers. (In some railcars, a centrally mounted cushion unit and sliding sill are used.)
Currently in common use on railcars are hydraulic cushion units which generally comprise a piston within a cylindrical barrel filled with a hydraulic fluid, typically oil. In general, the devices can be described as non-linear hydraulic shock absorbers. In a railcar collision, the piston is displaced through the barrel. As the piston travels through the barrel, the hydraulic fluid in the barrel is forced by the piston through orifices in the cylindrical wall of the barrel. The action of forcing the fluid through orifices acts to absorb impact energy by heating the fluid. Generally, the amount of force that can be translated into heat energy is proportional to the square of the piston velocity.
Typically, hydraulic cushion units are configured to absorb a constant force throughout the piston stroke by varying the number of orifices through which the fluid vents as the piston is displaced. More specifically, the orifices are distributed along the length of the barrel. Therefore, during the course of the piston's travel through the barrel, the piston bypasses orifices one (or more) by one, leaving fewer and fewer orifices through which the fluid can vent. When the force absorbed by a cushion unit is maintained substantially constant, the rate of change of acceleration is minimal. Thus, this configuration serves to minimize sudden changes in velocity or "jerking" motions of railcars connected in a train. After absorbing an impact, the piston is returned to its initial position in the barrel of the cushion unit by mechanical springs or a gas charged device.
In the typical operating environment, railcar cushion units are subject to failure, particularly in the hydraulic seals, from the wearing of moving parts and from rust and corrosion. Failure also can result from the stress of impacts greater than the rated capacity of the devices. To assure proper functionality of the devices, the performance of the devices is periodically tested.
Various test apparatus are known. U.S. Pat. No. 5,325,700 to Glen L. Litton (also the inventor of the present invention) discloses one railcar cushion unit tester designed to test railcar cushion units which are mounted on a railcar. This railcar cushion unit tester comprises a hydraulic ram which is electronically controlled to move a railcar cushion unit in various test motions. The tester also includes hydraulically actuated clamps mounted on adjustable arms to attach the tester at an end of a railcar whose cushion unit is to be tested.
Testing railcar cushion units while mounted in a railcar, however, tends to be time consuming. For each cushion unit tested, the tester is maneuvered into position next to the railcar containing the cushion unit. Then, the tester's arms are adjusted to properly position the tester's clamps for attachment to the railcar. The clamps are actuated to attach the tester to the railcar. Finally, the hydraulic ram and railcar cushion unit are coupled together. The testing of unmounted railcar cushion units adds even more time to each test for the cushion unit to be installed on a railcar configured to accommodate the cushion unit.